Abstract/Summary

Evidence of chemical recovery from acidification in European freshwaters has emerged in recent years, with many
previously damaged systems responding to decades of reduced acidifying emissions. Biological recovery, however,
has often lagged behind, and this has been ascribed to several possible mechanisms, including inertia in the food web.
We examined two decades of change in hindcasted food webs for Lochnagar, a Scottish mountain lake, to make
inferences about the potential dynamical stability of the system and to assess the prospects for future biological
recovery. Although community composition tracked temporal changes in acidity, this was neither sustained nor
directional, and mainly manifested as shifts in relative abundances rather than the establishment of more acidsensitive
species. The food web was highly interconnected and reticulate, especially in years when species richness
was low, and subsidized by external inputs of detritus. Among the primary consumers, generalist herbivore–
detritivores maintained feeding links with the scant algal resources, which appeared insufficient to support viable
populations of specialist grazers. Together, these characteristics, which are shared with many other acidified
freshwaters, are likely to make the community dynamically stable and resistant to invasions of potential new
colonists, thereby slowing the pace of future biological recovery.